Custom Post from custom form

Hi, I was wondering how can I make my own custom form that will make a custom post. I want to do this since I want to limit some of the options my users can use when making a post. I wan’t this basically outside the wordpress admin cp, and inside the blog. Where do I start?

Example: A form:
Author’s Username:
Topic:
Post Content:
Tag:
Content:

How can I do this? Any suggestions on where to start?